1. Field of the Invention
The present invention relates to an optical information recording medium for recording information by means of a laser beam having a high energy density.
2. Description of Prior Art
Information recording media for recording or reproducing information by the use of a laser beam of high energy density (i.e., optical information recording media) have been developed in recent years and are put into practical use. Such recording media are generally referred to as "optical discs" and have been widely utilized in various fields, for example, as an optical disc such as a video disc and an audio disc as well as a disc memory for a large-capacity computer and a large-capacity static image file, a micro-image recording medium, an ultramicro-image recording medium, a micro-facsimile, and an optical card.
As for the video disc or audio disc (i.e., compact disc or CD), optical discs for only reproducing information have been already put into practical use, and optical discs of DRAW-type (Direct Read After Write-type) for recording information have been also developed and partially put into practical use.
The information recording medium of DRAW-type basically comprises a disc-shaped substrate made of a plastic material or a glass material and a recording layer made of a metal such as Bi, Sn, In and Te or a semi-metal which is provided on the substrate. Writing (i.e., recording) of information on the optical disc can be carried out by irradiating the disc with a laser beam. The irradiated area of the recording layer of the optical disc absorbs energy of the laser beam and rise of temperature locally occurs, and as a result, a chemical or physical change (e.g., formation of pits) is caused to alter (or change) optical characteristics of the recording layer in the irradiated area, whereby information is recorded on the recording layer. Reading (i.e., reproduction) of information from the optical disc is also carried out by irradiating the disc with a laser beam, that is, the recorded information is reproduced by detecting a reflected light or a transmitted light corresponding to the change in the optical characteristics of the recording layer. The irradiation of the optical disc with the laser beam for writing and reading information is generally conducted on the pre-determined part of the disc surface. In order to guide the laser beam to accurately trace the pre-determined irradiation part (generally referred to as "tracking"), the surface of the optical disc is provided with a pre-groove of inverse-trapezoidal shape or other shape (i.e., tracking guide). Further, on the inner periphery side of the optical disc is generally formed pre-pits indicating pre-format information, such as address information, required for recording of signals.
As described above, the information recording medium of DRAW-type has a basic structure comprising a substrate, which is provided with a pre-groove and optionally provided with pre-pits, and a recording layer provided on the substrate.
As a material (i.e., recording material) for forming the recording layer of the information recording medium, the above-mentioned metals and dyes are generally known. An information recording medium using a dye as a recording material has such an advantageous feature in productivity that the recording layer can be easily formed on the substrate by a conventional coating method. Some metals or dyes among various recording materials for optical discs are known to be useful because of their high sensitivities, but optical discs using those metals or dyes show the reflectance of only 30 to 40%. Hence, a recording layer made of a dye generally has a drawback of low reflectance.
On the conventional information recording media of DRAW-type (optical discs of DRAW-type), information is generally recorded at a high linear speed, but a particularly high reflectance is not required therefor. On the other hand, compact discs of DRAW-type (CD-DRAW) on which information is recorded at a low linear speed using CD format signals requires a high reflectance. CD-DRAW having the high reflectance has such an advantage that the information recorded on the CD-DRAW can be reproduced using a commercially available CD player. The commercially available CD player is widely used as a reproducing player for CD having been recorded with music as information.
As a method of enhancing a reflectance, Japanese Patent Provisional Publication No. 1(1989)-242287 discloses a method of forming a dye recording layer using imidazoquinoxaline dyes, whereby a high reflectance can be obtained. However, when the signals recorded on the dye recording layer is reproduced using a laser beam, the reproduction characteristics are not sufficiently satisfactory. Moreover, the dye recording layer also has other problem that a read-error may occur with time in the reproduction stage.
An information recording medium which is provided with a metal reflecting layer on the dye recording layer to improve a reflectance is described in "Nikkei Electronics", p. 107, Jan. 23, 1989. In this information recording medium, as the dye recording layer absorbs a laser beam to melt the dye of the recording layer, the plastic substrate is heated and protruded on the dye recording layer side to form pits on the substrate.
European Patent Publication Numbers EP 0353393 A2 and EP 0353394 A2 disclose that an enhancing layer is provided between a dye recording layer and a metal reflecting layer to enhance a reflectance. As materials of the enhancing layer, inorganic materials such as SiO, SiO.sub.2, Si.sub.3 N.sub.4 and AlN are described.
In both of the former case (case of providing a reflecting layer on a dye recording layer) and the latter case (case of providing an enhancing layer and a reflecting layer on a dye recording layer) for improving a reflectance, the dye recording layer can be easily formed by a conventional coating method, while the reflecting layer and the enhancing layer are required to be formed utilizing a deposition method such as sputtering. Hence not only the cost becomes high because of the use of an expensive apparatus, but also the formation of those layers is time-consuming. The provision of the enhancing layer or the reflecting layer, therefore, results in disadvantages in the production.
In CD-DRAW, pre-format information such as address information required for recording signals is generally recorded by forming pre-pits on the inner periphery side of a substrate in the molding procedure of the substrate. However, in the case of providing a metal reflecting layer on a dye recording layer, the pre-pit signals become extremely small, and such small signals can be hardly reproduced. For this reason, it is impossible to provide a dye recording layer on the pre-pit area, and hence the production of optical discs is obliged to be complicated.